A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. In most cases, the process of exposing the target portions is repeated a plurality of times, thereby generating a device comprising a plurality of layers. In order for proper operation of the device, an accurate positioning of the layers relative to each other is required. As such, during the exposure process, the position of the substrate and relative to the patterning device needs to be known. In order to determine this position, a lithographic apparatus in general comprises a position measurement system such as an interferometer based measurement system or an encoder based measurement system. Such systems can e.g. be used for determining a position of a support e.g. holding a patterning device or substrate relative to a projection system of the apparatus.
In case an encoder based measurement system is used, such system may e.g. comprise one or more one- or two-dimensional gratings and one or more sensors co-operating with the grating or gratings. Such a system can e.g. be used for measuring the position of a substrate table relative to a projection lens by mounting the one or more sensors to the substrate table and mounting the one or more gratings to a reference frame such as a metrology frame to which the projection system is mounted. During use, the sensor or sensors may provide position signals representing the position of the sensor relative to the grating or gratings. When the position of the sensor relative to the substrate and the position of the grating relative to the projection system are known, the position signal can be converted to a position reference (e.g. suitable to be used by a position control system) representing the position of the substrate relative to the projection system. Such a conversion can e.g. comprise one or more look-up tables for converting the position signal from the sensor to a position reference. Such a look-up table or the like can e.g. be determined by an initial calibration of the position measurement system, which may e.g. involve determining overlay or focus maps or may involve the use of a second position measurement system.
As will be understood by the skilled person, an accurate positioning of e.g. the substrate relative to the projection system using the position measurement system as described, relies on the position reference determined from the position signals. However, due to deformations or displacements of either the sensor(s) or grating(s) (or both), the conversion as applied to the position signals may provide an inaccurate position reference which may thus result in an inaccurate positioning of e.g. the substrate during exposure of a target portion on the substrate. In order to at least partly compensate for such a drift, i.e. a deformation or displacement, a periodic calibration of the position measurement system could be considered. However, such a calibration will in general be time-consuming and would thus result in down-time of the apparatus, thus adversely affecting the productivity of the apparatus.